1. Field of the Invention
This invention relates generally to optical disk drives and more particularly to such drives that write data by laser heating of the data layer on the disk.
2. Description of the Prior Art
Optical disk drives provide for the storage of great quantities of data on a disk. The data is accessed by focusing a laser beam onto the data layer of the disk and then detecting the reflected light beam. Magneto-optical (M-O) systems write data by directing a laser to a spot on the data layer to heat it above its Curie temperature while the magnetic domain of the spot is oriented in either an up or a down direction by an external magnetic field. The data is read by directing a low power laser to the data layer. The differences in magnetic domain direction of the spots cause the plane of polarization of the reflected light beam to be rotated either clockwise or counterclockwise. This change in orientation of polarization of the reflected light is then detected. Phase change systems write data by directing the laser to a spot on the data layer to cause a structural change of the data layer, typically from a crystalline phase to an amorphous phase. The data is detected as changes in reflectivity as the laser beam passes over the different phases. Alloying systems write data by the heating of two chemically distinct materials, such as Bi.sub.2 Te.sub.3 and Sb.sub.2 Se.sub.3, to form an amorphous alloy in the data layer. In alloying systems the data is detected as changes in reflectivity. In all of these types of systems the writing of data thus occurs due to laser heating of the material in the data layer.
Pulse position modulation (PPM) and pulse width modulation (PWM) are two general ways to write data as marks on optical disks. In PWM, a mark can be either an individual spot or a series of overlapping or contiguous spots. PPM records information as the distance between the centers of the marks on the disk. PWM records information as the distance between the transitions or edges of the marks. A transition is either the beginning (leading) or end (trailing) edge of a mark. PWM recording is preferred to PPM recording because PWM recording is able to store more information in the same amount of space on the disk. However, PWM recording is more difficult to implement because the mark edges must be precisely positioned and written with sharp boundaries to ensure accurate recording.
One problem with both PPM and PWM recording in optical disk drives that write data by laser heating, especially at high data densities, is the thermal buildup that occurs within the disk during the laser writing process which can cause great distortions in the precise placement of the laser spots and mark edges. Thermal buildup occurs when there is insufficient time between the writing of successive spots to allow the data layer to cool. Under these conditions thermal preheating or cooling of the material in the data layer caused by the prior write history can lead to errors in the size and placement of the spots. This problem becomes more significant as the linear density of the spots increases and the spacing between the spots decreases because the peak temperature of this thermal interaction increases.
One proposed solution to thermal buildup is to adjust the laser pulse duration to achieve the correct mark length. Some systems use a series of highly pulsed laser beams to write contiguous or overlapping circular spots to form one long mark on the disk. However, these systems still experience some thermal buildup with the resulting inaccuracy in the placement of the mark edges and thus a blurring of the data transitions. Another proposed solution is to increase the local velocity of the disk beneath the laser spot so there is insufficient time for the heat to flow to an adjacent spot before that spot is written. However, this approach requires more precise focus and tracking servo systems and disks that have little axial and radial runout.
What is needed is a relatively simple optical disk drive that writes marks on the disk so that the mark edges are precisely aligned without the adverse effect of thermal buildup.